Wild fluctuations in California's winter and spring weather have hurt fragile butterfly populations, causing numbers to fall to the lowest in more than three decades and increasing the concerns of scientists about long-term declines linked to climate change and habitat loss.

UC Davis Professor Arthur Shapiro, considered one of the most prominent butterfly trackers in North America, said Monday he has found fewer butterflies this year than at anytime since he came to California 35 years ago.

"We have a severe depression of butterfly numbers at the lower elevations in Northern California, particularly in the Central Valley. We don't know if local populations are extinct or have dropped to low levels that we're unlikely to detect,'' he said.

Shapiro, an entomologist and professor of evolution and ecology, monitors 10 locations from Suisun Marsh to the Sierra Nevada and maintains one of the two largest butterfly databases in the world. The other is the British Butterfly Monitoring Scheme.

At most of the study sites, he has seen half or less than half the number of species typically present at this time in an average year. Near Vacaville at Gates Canyon in April 2005, he found 21 species and 378 individual butterflies. But last month he counted 10 species and 43 individual butterflies.

Many species already appear to be suffering from a serious long-term decline because of several factors, including changes in climate and loss of habitat, he said.

"This short-term anomaly has really kicked the populations while they're down and may have accelerated their decline,'' said Shapiro.

Species hit hard this year include the sooty wing, the large marble, the mourning cloak, Lorquin's admiral, the small checkered skipper, the sandhill skipper, the field skipper, the buckeye, the eastern tailed blue, the silvery blue and the migratory painted lady.

This is what Shapiro thinks is happening with many species:

The temperature in the state didn't drop enough to give the butterflies a certain amount of chilling, the cue to end their winter dormancy, be it in the form of larvae, pupae, egg or adult. They remained dormant and died because they couldn't take advantage of the food available during the one week of very warm weather in February in the Bay Area and Central Valley. The few that might have emerged in March probably died in the cold, wet conditions.

Jessica Hellmann, an assistant professor of entomology at the University of Notre Dame who researches butterflies throughout North America, has reviewed Shapiro's data and said it is critical in determining long-term changes in butterfly populations.

"We have similar observations for 2006 in California," Hellman said. "It is only because Art has 35 years of data that we can say 2006 is bad and is worse than it's been in a long time.

Hellmann and other scientists have published studies on checkerspot butterflies, showing, among other findings, that extinctions of two local populations were hastened by increasing variability in rain, a phenomenon predicted by global warming models.

Last year, the orange-and-black painted lady stunned Northern Californians by turning up in a migration of millions, if not billions. But this year, only a few painted ladies are known to have arrived, and earlier than normal, according to UC Davis scientists.

Painted ladies typically breed once in the late winter in the Mojave Desert, then in the Bay Area and the Central Valley and then in the Pacific Northwest, all in a year's time as the generations move north.

This year they appeared to have given up trying to breed in the southern deserts because of the unusually dry weather that didn't produce the plants that the butterflies needed in their caterpillar stage, scientists believe. They flew to Northern California earlier than usual and tried to breed with no apparent success, Shapiro said. He doesn't know yet whether they reached the Pacific Northwest.

"There doesn't appear to be any organized migration on the west side of Sierra,'' he said, adding that he has seen only one painted lady this year in the Sacramento-Davis area and has received reports of only three others in the area. But he cautioned that just because they're not here doesn't mean there aren't painted ladies elsewhere. This particular species typically expands in some areas while contracting in others, he said.

Six feet of snow still blankets parts of the Sierra, so Shapiro hasn't been able to count butterflies on the 7,000-foot Donner Summit or the 9,000-foot Castle Peak north of Donner Summit. Over the years, he has found the greatest number of butterfly species -- 115 -- at Donner Summit.

This year's anomalous late arrival of butterflies goes against the longer-term trend. Many species this year are running four to six weeks later than normal instead of the three weeks earlier that his long-term data show, he said.

Based on his long-term database, the analysis of 23 species over 31 years found that many of the butterflies are coming out earlier in the spring than in the past. Shapiro and one of his students, Matt Forister, correlated the earlier appearance with trends in the weather data in the Sacramento-Davis area.

For those species that had a statistically significant earlier appearance, the average shift was 24 days earlier. Any shift can disrupt the butterflies' survival. There's a synchronicity in nature, and many butterflies need to have certain plants available during a certain time in their life cycle.

Shapiro said that for many years he "pooh-poohed the evidence that butterfly populations were going downhill. But all that changed in 1999, when a whole bunch of low-elevation species showed an unmistakable drop-off, and the decline has continued.''

But he remains optimistic that the butterflies will survive. "Butterflies have been around for 40 or 50 million years,'' he said, "so they've been through it before.''

The painted lady

Painted ladies breed on desert annuals in Death Valley, then migrate north to breed again in the Bay Area and Central Valley.

This year, the dry desert produced few plants, and the butterflies apparently stopped breeding. Only a few have been seen in Northern California.

Hardest hit species of butterflies

Scientists blame the state's wild weather in 2006 for the worst year for butterflies in 35 years. UC Davis scientists are seeing half or less than half the number of species present at this time in an average year and far fewer individuals. The mild winter disrupted the lifecyles of some species, and the resulting change in the food supply affected others.